WASHINGTON — Three weeks after President Obama hailed a landmark deal to suspend most of Iran's nuclear program for the next six months, the mood among U.S. officials about the next round of negotiations has shifted from elated to somber, even gloomy.

"I wouldn't say [chances of success are] more than 50-50," Obama said last week. U.S. officials are "very skeptical" that Iran will accept Western demands, said his lead negotiator, Wendy R. Sherman.

The shift, officials say, is the result of a growing recognition of the compromises each side must make to resolve the decade-old impasse over Western suspicion that Iran will someday try to build nuclear weapons and the Iranian demand that the sanctions crippling its economy be lifted.

Problems already have emerged. Technical talks in Vienna aimed at implementing the initial deal stopped Thursday when Iranian negotiators unexpectedly flew back to Tehran, reportedly in response to the Obama administration's decision to expand its blacklist of foreign companies and individuals who have done business with Iran in violation of sanctions.

On Sunday, Iranian Foreign Minister Mohammad Javad Zarif told CBS' "Face the Nation" that he had not been given notice of the U.S. action, but that Iran's unhappiness over the blacklisting was not likely to scuttle the talks.

"The process has been derailed," Zarif said. "The process has not died. We are trying to put it back and to correct the path and continue the negotiations, because I believe there is a lot at stake for everybody."

Even before Thursday's interruption, experts had struggled to determine how to sequence the complex next steps involved: neutralizing a stockpile of medium-enriched uranium and freezing most other enrichment operations in exchange for granting Iran access, in installments, to $4.2 billion of its own funds held in banks overseas and easing sanctions on petrochemical and auto exports.

Crafting a comprehensive agreement that satisfies powerful political constituencies in Iran as well as in Washington and its negotiating partners — Britain, China, France, Germany and Russia — will be far more challenging. Yet the contours of a potential deal are clear.

"We can now identify all the main issues involved in the final deal," said David Albright, a former United Nations nuclear inspector who heads the nonpartisan Institute for Science and International Security in Washington. "That doesn't mean we can resolve them."

When talks resume next month, "we still don't know which Iran is going to show up: the one that cooperates or the one that nickels and dimes you," Albright said.

White House officials want Iran to scrap its prized Arak heavy-water reactor, which is still under construction, and abandon a fortified underground uranium-enrichment facility at Fordow. They also want Iran to fully disclose long-hidden nuclear research and activities at the Parchin military site and open itself to intensive monitoring by United Nations inspectors.

Iran also would be required to accept strict controls on imports of so-called dual-use materials, a broad array of components and supplies that could be viable either in a nuclear arms program or for civilian purposes. Imports of high-strength steel and carbon fiber, which are used in hundreds of everyday products, could be restricted.

In turn, Tehran seeks to continue enriching uranium at least to levels suitable for generating electricity, producing medical isotopes and filling other commercial needs. And it wants a lifting of the oil, financial, shipping and other sanctions that have pushed its economy into deep recession and largely cut it off from international energy markets and banking systems.

Even more difficult perhaps, the two sides will need to agree on how long any deal should be enforced. The comprehensive accord sometimes is called a "final" or "permanent" deal, but the Iranians want to keep it in force for less than five years. U.S. officials are expected to seek a deal for 20 years or longer.

The final line in the interim agreement signed last month in Geneva says Iran ultimately should "be treated in the same manner" as any other signatory to the international Nuclear Nonproliferation Treaty, which seeks to prevent the spread of such weapons and promote the peaceful use of nuclear energy. The treaty obligations are far less restrictive than those likely under a comprehensive deal, however.

"So for sure the Americans will want this to be a very long agreement, and the Iranians a very short one," said Brookings Institution senior fellow Robert Einhorn, who was a member of Obama's Iran team until May.

In public comments Dec. 6, Obama said he could envision a comprehensive deal that "involves extraordinary constraints and verification mechanisms and intrusive inspections, but that permits Iran to have a peaceful nuclear program."

Defining "peaceful nuclear program" may be the toughest challenge of all.

Iran insists it will not surrender its ability to continue low-level enrichment on its soil. Obama says Iran's nuclear program is so far along that it is unrealistic to expect all enrichment to be abandoned.

At least in theory, Obama said, Iran could be granted "some modest enrichment capability" that is so restricted and so carefully monitored that the country, as a practical matter, does not have "breakout capacity," meaning an ability to suddenly race to build a bomb.

U.S. officials would like Iran to reduce its inventory of first-generation centrifuges at its chief enrichment site, Natanz, to perhaps 6,000 from 19,000. That, at the least, would stretch the time Iran might need to obtain the fissile fuel for a weapon.

Iranian officials, perhaps trying to stake out a tough negotiating position, say they also want to keep the bomb-resistant underground enrichment plant at Fordow and the heavy-water research reactor at Arak. But if Arak came online, it would produce plutonium that could provide Iran a second source of fuel for a nuclear bomb.

Iran has hinted it might give up Arak, which has been plagued by technical problems, perhaps in exchange for a Western light-water reactor. Officials have also said they might be willing to convert Fordow to research uses.

Cutting the number of first-generation centrifuges at Natanz might prove more problematic. Iran is determined to develop an industrial program to enrich uranium to low levels of purity and expects it will be free to pursue that commercial goal after a nuclear deal is signed.

"It's hard to believe that Iran would easily accept the idea of just 6,000 centrifuges," Albright said. "They'll fight rollback."

More important, perhaps, Iranian officials say that any long-term deal must include an end to all U.S. nuclear-related sanctions. That would require repeal by Congress, and many lawmakers at recent House and Senate hearings were deeply critical of the interim deal, especially because it allows Iran to continue enrichment.

Cliff Kupchan, a former State Department official now at the Eurasia Group consulting firm, said that asking Congress to roll back sanctions set "an extraordinarily high bar."

paul.richter@latimes.com